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A full-sized forerunner to the 2004 Indian Ocean tsunami hit the region between 500 and 700 years ago, a new study suggests.

Two groups of geoscientists report evidence today in the journal Nature that a similar event occurred around 1400 AD.

Until now there has been no historical record of a predecessor to the Indian Ocean tsunami and the associated magnitude 9.2 associated earthquake. More than 220,000 people throughout southern Asia and as far away as the east coast of Africa died in the disaster.

Amy Prendergast from Geoscience Australia in Canberra, Australia and colleagues studied the sedimentary record on an island north of Phuket, Thailand to help pin point the date of ancient tsunamis.

The 2004 tsunami rose to as much as 20 metres above sea level at Phuket, and ran two kilometres across a grassy beach ridge plain, where it left a sand sheet between five and 20 centimetres thick.

Dating material

To establish the date of the ancient tsunamis the researchers measured the amount of radioactive carbon in the leaf matter contained in the sand sheet, as well bark fragments in the uppermost centimetre of underlying soil, Prendergast says.

"We dug pits and augered holes at more than 150 sites to study the sub-surface sediments and construct stratigraphic cross-sections," she says.

The team studied the sedimentary structures and grain size in the tsunami-deposited sand sheets, which told them about the flow direction of the tsunami.

They also looked at microfossils called diatoms, which showed the sediments came from the offshore marine environment.

The other report in Nature analysed sedimentary evidence from Aceh, in northern Sumatra and also found evidence for the earlier tsunami.

While both teams were able to identify evidence of other ancient tsunamis, it was only the event from 1400 AD that could be correlated in both areas.

Future events

Prendergast says the geological record of tsunamis from Thailand gives tangible evidence that several large tsunamis have occurred in the Indian Ocean during the last 2,500 years.

"Whilst these events are infrequent, the record suggests that such events are likely to occur in the future," she says.

"If the geological records the Indian Ocean region had been well studied prior to 2004, the tsunami hazard may have been recognised and its devastating impact may have been reduced."